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   Mountain Shadow Formation 

   A cubic mountain and its shadow. When the sun is very low the shadow can be a hundred miles or so long. It is a three dimensional void of dark air rather than an ordinary 2D shadow cast on the ground or clouds. It tapers slightly because the sun is a disk rather than point source.    The shadow as seen from the cubic mountain's flat top. Perspective makes the immensely long square cross section void appear triangular in outline. The shadow's end, the only real cue to the mountain shape, is so far away that it is small and indistinct.
  Mountain shadows at sunrise and sunset are immensely long tunnels of unlit air, crepuscular rays in fact.

From the summit, perspective effects nearly always make the shadow triangular regardless of the mountain's profile. You are standing at the top edge of the shadow tunnel and looking out along its length which can be more than a hundred miles. Only the shadow's end carries much information about the mountain shape and it is so far away and in any event blurred by the 0.5º angular spread of the sun's rays that it is hardly visible.